xen/pci: Make use of the helper macro LIST_HEAD()

Replace "struct list_head head = LIST_HEAD_INIT(head)" with
"LIST_HEAD(head)" to simplify the code.
Signed-off-by: Cai Huoqing <cai.huoqing@linux.dev>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20220209032842.38818-1-cai.huoqing@linux.devSigned-off-by: Juergen Gross <jgross@suse.com>

drivers/xen/pci.c

@@ -264,7 +264,7 @@ struct xen_device_domain_owner {
 };
 
 static DEFINE_SPINLOCK(dev_domain_list_spinlock);
-static struct list_head dev_domain_list = LIST_HEAD_INIT(dev_domain_list);
+static LIST_HEAD(dev_domain_list);
 
 static struct xen_device_domain_owner *find_device(struct pci_dev *dev)
 {